CMAQ Ecosystem Exposure Studies

Chesapeake Bay

Chesapeake Bay is the largest estuary in the United States and was the nation’s first estuary targeted by Congress for restoration. Reversing the rapid loss of living resources due to excess nutrients (mainly nitrogen) and restorating the quality of the Bay, has been the goal of the Chesapeake Bay Program since it began 1983.

Atmospheric deposition of nitrogen to the Chesapeake Bay watershed and Bay surface is a major contributor to the Bay nitrogen load, affecting current conditions and which need to be addressed in Bay restoration efforts. The atmosphere is estimated to contribute a quarter of the total nitrogen load delivered from the watershed to the Bay. Direct atmospheric deposition to the Bay’s tidal waters increases the fraction of the total load of nitrogen to the Bay from atmospheric deposition to about a third.

Chesapeake Bay has been placed on EPA’s list of impaired waters, with a total maximum daily load (TMDL) plan required in 2011 with implementation in 2015. To provide the best modeling science for the TMDL plan, a major upgrade of the Chesapeake Bay Watershed model, version 5.2, is being used as well as CMAQ5.0. This atmospheric modeling will be a major update from earlier use of CMAQ4.7. The grid size is 12km, better resolving the Bay, and the effect of sea salt is included. In a major scientific advance, CMAQ5.0 also accounts for the bi-directional air-surface exchange of ammonia. The CMAQ modeling for the Chesapeake Bay TMDL planning has three major foci:

Development of scenarios estimating the deposition reductions expected by 2017 and 2030 due to Clean Air Act regulations, such as the Clean Air Interstate Rule (CAIR) that was further modified as the result of court actions to be replaced by the Cross State Air Pollution Rule (CSAPR)

A new NH3 budget analysis at 12km, using a prototype CMAQ with NH3 bi-directional air-surface exchange incorporated, showed that incorporating bi-directional exchange of ammonia will have an important impact of reducing the local dry deposition and an impact on the estimates of the range of influence of ammonia emissions, almost doubling the range.

Estimation of the relative contribution the nitrogen oxide (NOx) emissions from the six Bay states make to the atmospheric deposition of oxidized nitrogen to the Bay watershed and Bay surface after implementation of (CAIR). The Bay state emissions are also subdivided into the major emission sectors such as power plants, industry and on-road vehicles. The state allocation data form the basis for a management decision rule for allocating state emission reductions that are beyond the national rules to watershed deposition reductions that can count as state allocation reduction credits.

Model-predicted contributions of six bay states account for 50% of the 2020 oxidized nitrogen deposition to the Chesapeake Bay Watershed